The power required by the hoisting machine of an elevator varies depending on factors including load, speed, traveling direction and phase of the operating cycle of the elevator. It is advantageous to keep the power requirement as small as possible to minimize both the size of the hoisting machine and the required mains connection size. A traditional solution designed to minimize the power needed to move an elevator car is to provide each elevator in the elevator system with a counterweight, which typically is so dimensioned that its mass corresponds to about 50% of the mass of the elevator car with full load. When the elevator is driven in the heavier direction, i.e. when an elevator car with a load above 50% is driven upwards or an elevator car with a load below 50% is driven downwards, the main direction of power transfer is from the electricity network towards the elevator motor. The largest instantaneous power is needed at the beginning of the operating cycle as the speed of the elevator car is being accelerated. When the elevator is driven in the lighter direction, the potential energy of the elevator car-counterweight combination is reduced, and the elevator motor converts mechanical energy into electric energy. The power generated when the elevator is being driven in the lighter direction or braked can be either dissipated in separate resistor packs or it can be fed back into the electricity network. Solutions are also known according to which in elevator groups comprising several elevators the power generated by one elevator can be utilized for driving other elevators comprised in the same elevator group in the heavier direction. However, supplying the power thus generated to other elevators in the elevator system requires that the starting order and traveling directions of elevators loaded in different ways be optimized so as to ensure that the energy flows in the system are in balance at each instant. This is not possible in all operating situations in the elevator system, in which case the power generated may have to be dissipated in resistors or in some other similar way. Another prior-art solution is to use energy storages in conjunction with the hoisting machines of an elevator system to allow the electric power produced by the elevator system to be stored so that it will be later available to the elevators comprised in the system. For example, specification US2003/0089557 A describes a system in which the power taken by an elevator system from the electricity network can be reduced by connecting supercapacitors and batteries to the power supply equipment of the elevator system. In this system, supercapacitors are used to smooth out instantaneous power peaks at the beginning of the operating cycle, and batteries are needed to reduce the required average power.
Using a counterweight in conjunction with each elevator car takes up building space that could often be advantageously used for other purposes. By omitting the counterweight, it is possible e.g. to accommodate a larger elevator car in an elevator shaft of a given size than in the case of elevators with counterweight. New efficient hoisting machine solutions have made it possible to increase the power of the elevator hoisting machine without unreasonably increasing the size of the hoisting machine, and the use of elevators without counterweight is gaining ground. In elevator systems having no counterweight, the power requirement of an elevator traveling in the heavier direction is greater than in counterweighted elevator systems. Correspondingly, when the elevator car is moving downwards, an elevator without counterweight produces more energy than a counterweighted elevator does. Large power transfers between the electricity network and the elevator system increase the requirements regarding the power supply as both the rated power and the harmonics content of the voltage and current is increased. Filters provided in the mains inverter of elevator systems are expensive when designed for high powers. It may also happen that the internal electric network of the building can not receive the power produced by elevators without counterweight, in which case the voltage in the internal electric network of the building will rise. When a building is to be provided with several elevators, as an elevator group or otherwise, the connection power required by the elevators easily increases to a level that makes it unreasonable to use elevators without counterweight in the building, although they offer a significant space saving.
By connecting energy storages to the electricity supply of the elevator e.g. in the manner indicated by specification US2003/0089557, a proportion of the energy produced by an elevator without counterweight during downward travel can be stored for later consumption. However, as the power generated by an elevator without counterweight is considerably greater than that produced by a counterweighted elevator, the size of supercapacitors needed to store the energy produced would increase significantly in the case of an elevator without counterweight, so the energy storage would be expensive and take up a large space. Furthermore, the service life of supercapacitors is limited, typically about 30 000 hours, and, due to leakage currents, they are particularly well suited only for short-term storage of energy. Optimization of elevator running schedules and prior-art electric energy storage solutions can not be regarded as offering an optimal solution for minimization of the size of the electric network connection of non-counterweighted elevator systems.
Specification U.S. Pat. No. 5,712,456 discloses an elevator system comprising one elevator and including a flywheel for storing the energy of the elevator.
Specification U.S. Pat. No. 5,936,375 discloses a hoisting equipment that comprises a flywheel used as an energy storage. The hoisting equipment according to this specification comprises one hoisting device. Moreover, the equipment comprises a flywheel and a motor and a power converter for controlling the flywheel.
In addition, specification U.S. Pat. No. 4,657,117 discloses an elevator system in which energy produced by one elevator is stored in a flywheel. The control apparatus controlling the elevator motor in this system is a Ward Leonard drive.
If the use of an elevator's energy storage is limited to one elevator, implementing an energy storage in an elevator system comprising a plurality of elevators will be complicated in practice. In that case each elevator needs a separate energy storage as well as separate equipment for the transfer of energy between the elevator motor and the energy storage.